The channel electron multiplier (CEM) is a well known device. The CEM consists of an elongate tube of material which is a secondary emitter of electrons. Conventionally, this secondary electron emitter material is carried on the inner surface of a structural tube formed of insulative material. An electric current, and associated electrostatic field in the secondary electron emitter material, is maintained along the length of the CEM from an inlet end to an outlet end of the tubular CEM structure. Because the performance of the conventional CEM depends on length-to-diameter ratio rather than physical size of the structure, the channel size can be reduced to very small dimensions.
Accordingly, arrays of plural channel electron multipliers have been fabricated. A conventional device which provides such an array of channel electron multipliers is the microchannel plate. Conventionally, microchannel plates are made by drawing a number of fine-dimension glass tubes of either a hollow configuration or of a configuration with a removable core fiber. The glass tubes are joined in bundles and further drawn under pressure causing the tubes to bond to one another at their outer surface, thus forming a boule or elongate rod-like structure of multiple fine-dimension glass tubes in parallel. Next, a pair of spaced apart parallel transverse cuts across this boule of glass tubes defines between the cut lines a comparatively thin plate having perhaps a million glass tubes extending between its opposite faces. If the glass tubes are of fiber core construction, the core fiber is etched out using chemicals. For example, an acid or a base may be used to etch the glass.
The inner surface of each of the multitude of glass tubes (or microchannels) is then activated to make this glass surface a practical secondary emitter of electrons. This activation of the glass surface is effected by reducing this surface at elevated temperature in a hydrogen atmosphere. The glass of the tubes is made of a material which is doped with selected materials, such as lead and antimony. After reduction of the glass, this doped glass leaves metal atoms or metal oxide molecules exposed on and close to the inner surface of the microchannels, and provides a thin coating of glass semiconductor material extending along the inner surface of the microchannels between the opposite faces of the plate.
A metallic electrode is applied to each of the opposite faces of the plate, and the microchannel plate is operated with an applied electrostatic potential applied across these electrodes. As a result, a current flow between the electrodes takes place in the surface layer of reduced semiconductor glass, which current is referred to as the "strip current" of the microchannel plate. Because the glass of the tubes is itself an insulator with a bulk-resistivity in the range of 10.sup.17 to 10.sup.22 ohm-cm, substantially no practical electric current flows in the body of the microchannel plate itself--other than in the semiconductor reduced-glass coating each of the microchannels. The microchannel is operated in an evacuated environment of reduced pressure to allow electron flow along the channels with amplification by secondary emission of electrons from the inner surfaces of the microchannels.
The microchannels in a conventional microchannel plate are straight, and hence are subject to ion feedback. The ion feedback occurs because molecules and atoms of residual gas and other materials in the operating environment of the microchannel plate, and which become positively charged, are accelerated by the applied electrostatic field in a direction opposite to the electron flow. Because these ions are both very massive compared to an electron, and are accelerated to a high potential energy by the applied electrostatic field, they can be destructive to surfaces which they impact, and the impacting ions can cause unwanted emissions of electrons from the microchannel walls and/or the photocathode. As is known in the technologies using microchannel plates, these ions flowing toward a photocathode, for example, can both erode the photocathode by their dynamic impact, and also may imbed into the cathode, thus changing the crystalline structure and chemistry with resulting loss of performance of the photocathode to liberate photoelectrons in response to incident photons of radiation.
For these reasons, conventional microchannel plates have been operated in pairs with the microchannels of the paired plates forming a chevron shape to trap ions feeding back toward the inlet end of the first microchannel plate. Unfortunately, it is impossible to precisely align the microchannels of one plate to those of the other, so that resolution of paired microchannel plates is always less than one plate alone could provide. Alternatively, a few microchannel plates have been formed with curved channels in order to impact the ions with the walls of the channels, and thereby recombine the ions with an electron to produce neutral particles. However, microchannel plates with curved channels are very expensive and difficult to manufacture.
Conventional devices which use microchannel plates are image intensifier tubes of night vision systems, and photomultiplier tubes. Photomultiplier tubes are used for such purposes as scintillation detectors in particle accelerators and fluoroscopic detectors of chemical analyzers. A night vision system converts available low intensity ambient light to a visible image. Such night vision systems require some residual light, such as moon or star light, in which to operate. The star-lighted sky of the night is generally rich in infrared radiation, which is invisible to the human eye. The infrared ambient light is intensified by the night vision scope to produce an output image in light which is visible to the human eye. The present generation of night vision scopes use image intensification technology with a photocathode responsive to both visible and infrared photons to release photoelectrons. One or more microchannel plates are used to amplify the low level of photoelectrons to render a shower of secondary-emission electrons in a pattern replicating the invisible infrared image. These electrons are directed onto a phosphorescent screen to provide a visible image.
Alternatively, a microchannel plate can be used as a "gain block" in a device having a free-space flow of electrons. That is, the microchannel plate provides a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern. Such a device is useful as a particle counter to detect high energy particle interactions which produce electrons.
Regardless of the data output format selected, the sensitivity of the image intensifier or other device utilizing a microchannel plate is directly related to the amount of electron amplification or "gain" imparted by the microchannel plate. That is, as each photoelectron enters a microchannel and strikes the wall, secondary electrons are knocked off or are emitted from the area where the photoelectron initially impacted. The physical properties of the walls of the microchannel are such that, generally and statistically speaking, a plurality electrons are emitted each time these walls are contacted by one energetic electron. In other words, the material of the walls has a high coefficient of secondary electron emissivity or, put yet another way, the electron-emissivity of the walls is greater than one.
Propelled by the electrostatic field across the microchannel plate, the secondary-emission electrons travel along the microchannels toward the far surface of the microchannel plate and away from the photocathode and point of entry of the photoelectrons. Along the way, each of the secondary-emission electrons repeatedly impacts with and interacts with the walls of the microchannel plate resulting in the emission of yet more secondary-emission electrons. Statistically, some of the photoelectrons and secondary-emission electrons are absorbed into the reduced glass semiconductor material at the inner surface of the microchannels so that generally not all of the secondary-emission electrons escape the plate at the exit end of the microchannels. However, the secondary electrons continue to increase or cascade along the length of the microchannels.
The number of electrons emitted thus increases geometrically along the length of the microchannel to provide a cascade of electrons arising from each one of the original photoelectrons which entered the microchannel. As discussed above, this electron cascade, in a pattern which replicates the initial pattern of photoelectrons, then exits the individual channels of the microchannel plate and, under the influence of another electrostatic field, is accelerated toward a corresponding location on a display electrode or phosphor screen. The number of electrons emitted from a microchannel, when averaged with those emitted from the other microchannels, is equivalent to the theoretical amplification or gain of the microchannel plate.
While the intensity of the original image may be amplified several times, various factors can interfere with the efficiency of the process thereby lowering the sensitivity of the device. For example, one inherent problem of microchannel plates is that a photoelectron released from the photocathode may not fall into one of the slightly angulated microchannels, but may impact the bluff conductive face of the plate in a region between the openings of the microchannels. Electrons that hit the conductive face are likely to produce low energy secondary electrons or high energy reflected electrons which move back toward the photocathode for a distance before being returned by the applied electrostatic field to the microchannel plate and into a microchannel. Such photoelectrons then produce in a microchannel spaced from their initial point of impact a number of secondary electrons. These secondary electrons are issued from a part of the microchannel plate not aligned with the proper location of photocathode generation, and decrease the signal-to-noise ratio as well as visually impairing the image produced by an image intensifier tube. Other times the errant electron is simply absorbed by the conductive face of the plate and is not amplified to produce part of the image or signal produced by the detector anode. Such an electron loss reduces the effective gain and signal-to-noise ratio of a microchannel plate.
Of course, one solution to this problem is to increase the amount of microchannel aperture area and reduce the amount of bluff surface area on the input face of the microchannel plate as was done in U.S. Pat. No. 4,737,013, issued Apr. 12, 1988, to Richard E. Wilcox. Through the use of an etching barrier around each microchannel these particular microchannel plates have an improved ratio of total end open area of the microchannels to the active area of the plate (i.e., an improved open area ratio, "OAR", at the inlet ends of the microchannels). Specifically, the etching barrier incorporated in the plate allows more precise etching of the microchannel tubes in the plate. The technique allows the plates to be produced with a theoretical open area ratio (OAR) of up to 90% of the plate active surface. As a result, the photoelectrons are not as likely to miss one of the microchannels and impact on the face of the microchannel plate to be bounced into another one of the microchannels. This higher OAR improves the signal-to-noise ratio of image intensification.
While the OAR may be improved using conventional methods, other factors still reduce the gain and decrease the signal-to-noise ratio of the conventional microchannel plate. In particular, coating the input face of the conventional microchannel plates with a conductive metallic electrode material significantly reduces the gain provided by a microchannel plate. Generally, the conductive metallic electrode materials on a statistical basis have an electron-emissivity coefficient of less than unity (i.e., less than one). More particularly, conventional deposition procedures for these metallic electrodes entail rotationally disposing the microchannel plate so that the axis about which the microchannel plate is rotated parallel the axis of the microchannels. A deposition source for the metallic electrode is thus angularly disposed relative to the axis of the microchannels at a distance for the input face of the plate. As the microchannel plate is rotated, metallic material is evaporated from the source onto the microchannel plate.
Because the microchannel plate rotates about an axis which is parallel with the axis of the microchannels, the metallic material from the source coats not only onto the input face of the microchannel plate, but also for a distance into the microchannels themselves. The distance into the microchannels to which the metallic electrode material will coat is dependent upon the angulation of the source with respect to the axis of the microchannels themselves in essentially a line-of-sight process. Because the microchannel plate is rotated about an axis parallel to the axis of the microchannels, the depth of metallic coating penetration into the microchannels is substantially uniform circumferentially about the microchannels.
As a result, in addition to covering the face of the microchannel plate, the conductive electrode coating extends into the individual microchannels of the plate, covering a substantial portion of the entrance surface portion of each microchannel which would be visible (on a microscopic scale) if one were to look into the microchannels perpendicularly to the face of the plate. Accordingly, while conventional processes and methods for deposition of the metallic conductive electrode material renders the parallel faces of the microchannel plates sufficiently conductive, the unavoidable coating of at least a part of the inner entrance surface portion of the microchannels themselves unavoidably interferes with amplification of photoelectrons due to the low electron-emissivity coefficient of the metallic coating material.
However, the solution to this problem of lost gain is not as simple as simply increasing the length of the microchannels so as to extend the length over which the secondary electron emission process is effective. Initially, it would seem that the gain of a microchannel plate could be increased indefinitely simply by making the plate thicker. However, a microchannel plate cannot simply be made thicker because doing so severely and adversely affects the signal-to-noise ratio of the microchannel plate. The reason for this prohibition against increasing the thickness of a microchannel plate to increase its gain can be understood when one considers the statistical effects involved in emission of secondary electrons within the microchannels.
Each time an electron impacts the wall of a microchannel, there is a probability of the electron causing the emission of one or more secondary electrons. For the metallic electrode material, which is on the entrance portions of the microchannels of conventional microchannel plates, this probability coefficient is about unity (i.e., 1). Thus, there is some electron signal loss and loss of amplification length for the microchannel plate because of this metallic electrode material at the entrance portion of the microchannels. For the material along substantially the remaining length of the microchannels, the secondary electron-emissivity is greater than one, and the statistical process results in an increase in the number of electrons moving along the channels from the entrance end to outlet end. However, each time an electron impacts the walls of a microchannel, there is also the statistical probability that a positive ion will be released. When a positive ion is released, it travels in the opposite direction to the electron flow along the microchannel because of the prevailing electrostatic field. As a positive ion travels toward the entrance end of a microchannel, it also will impact and interact with the walls of the channel. Similarly to an electron, a positive ion has a probability of causing emission of secondary electrons.
Secondary electrons which are emitted because of positive ions moving toward the inlet end of a microchannel plate and impacting the channel walls represent noise in the output of the microchannel plate. A point of diminishing returns is reached if a microchannel plate is increased in thickness beyond a certain length-to-diameter ratio for the microchannels. Further increase in the thickness of the microchannel plate results in little or no increase in gain because of space-charge saturation. If the voltage across the microchannel plate is increased to overcome the space-charge saturation limit, the probability of emission of positive ions increases faster than the emissivity of electrons. As a result, the signal-to-noise ratio of the thicker microchannel plate is severely decreased.
One proposed solution to this problem is presented in U.S. Pat. No. 3,742,224, issued Jun. 26, 1973 to Bernard C. Einstein. According to the '224 patent, a microchannel plate is understood to be provided with a substantially optically transparent but not self-supporting aluminum coating provided at the inlet end of the microchannels. This coating spans the inlet ends of the microchannels, and is asserted to trap positive ions.
Of interest is a paper entitled, "Preliminary Results with Saturable Microchannel Array Plates", by J. G. Timothy, published in Review of Scientific Instruments, Volume 45, No. 6, June 1974, pp. 834-837. This paper investigates the performance of microchannel plates having a lateral field caused by purely axial current flows in the secondary electron emitting semi-conductor surface of a microchannel plate provided with axially extending insulative strips separating this surface circumferentially into a plurality of elongate strips. The performance of such microchannel plates was not entirely satisfactory because of charge accumulation on the insulative strips. However, the author speculates that a different internal configuration for the microchannel plate might be tried with either a higher surface conductivity or a bulk-conductivity. How these speculative microchannel plates are to be realized is not taught by the author. There is no suggestion in this paper that the reduced secondary electron emitting surface portion can be omitted, or that a bulk-conductive glass will itself provide an adequate level of secondary electron emissions without reduction. Additionally, no appropriate bulk-conductivity glasses were then available in the microchannel plate, image intensifier, or photomultiplier arts.